Arp 220: A Post-starburst Galaxy with Little Current Star Formation outside of Its Nuclear Disks
Bibliographic record
Abstract
Abstract The ultraluminous infrared galaxy Arp 220 is a late-stage merger with several tidal structures in the outskirts and two very compact, dusty nuclei that show evidence for extreme star formation and host at least one active galactic nucleus (AGN). New and archival high-resolution images taken by the Hubble Space Telescope provide a state-of-the-art view of the structures, dust, and stellar clusters in Arp 220. These images cover the near-ultraviolet, optical, and near-infrared in both broad- and narrowband filters. We find that ∼90% of the Hα emission arises from a shock-ionized bubble emanating from the AGN in the western nucleus, while the nuclear disks dominate the Paβ emission. Four very young (∼3–6 Myr) but lower-mass (≲104 M ⊙) clusters are detected in Hα within a few arcseconds of the nuclei, but they produce less than 1% of the line emission. We see little evidence for a population of massive clusters younger than 100 Myr anywhere in Arp 220, unlike previous reports in the literature. From the masses and ages of the detected clusters, we find that star formation took place more or less continuously starting approximately a few gigayears ago with a moderate rate between ≈3 and 12 M ⊙ yr−1. Approximately 100 Myr ago, star formation shut off suddenly everywhere (possibly due to a merging event), except in the nuclear disks. A very recent flicker of weak star formation produced the four young, low-mass clusters, while the rest of the galaxy appears to have remained in a post-starburst state. Cluster ages indicate that the tidal structures on the west side of the galaxy are older than those on the east side, but all appear to predate the shutoff of star formation. Arp 220 has many of the characteristics expected of a “shocked post-starburst galaxy,” since most of the system has been in a post-starburst state for the past ∼100 Myr and the detected Hα emission arises from shocked rather than photoionized gas.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".